This is what you get if you take a photo of the Moon every night for a month, then make a movie out of those pictures. The Moon’s phases aren’t surprising, but the Moon also appears to grow and shrink as it orbits the Earth. This happens because the Moon’s orbit is slightly elliptical; its distance to the Earth varies by about 10%.

Also, the Moon appears to “wobble” from left to right. That’s because the Earth’s gravity pulls harder on the Moon the closer it is to the Earth, so the Moon travels faster in its orbit when it’s closer to the Earth. The Moon’s rotation rate matches its average orbital speed (which is why we only see one face of the Moon), but its orbital speed varies during the orbit while its rotation rate remains fixed, so the Moon appears to wobble from left to right.

2009-08-10 Update: I’ve noticed that many people arrive at this article by googling moon wobbles. All the other sites on the first page claim that “moon wobbles” are responsible for explosions, mass-murder, earthquakes, terrorism, etc. Sadly, I need to emphasize that the wobbling I’m describing can’t possibly result in these kinds of ludicrous effects.

The Moon is spirallingaway from Earth due to tidal friction, which slows Earth’s rotation. I used to think tidal braking would stop when Earth’s daysidereal day, to be specific. matched the Moon’s orbital period, as with Pluto and Charon. (more…)